That works at point blank range, but if we calculate the damage rolls throughout the range increments, I don't think it will scale as smoothly.
[spoiler]1st range increment
2d10 damage: min 2, max 20 (avg 11)
5d4 damage: min 5, max 20 (avg 12.5)
2nd range increment
2d10-1 damage: min 1, max 19 (avg 10)
5d4-1 damage: min 4, max 19 (avg 11.5)
3rd range increment
2d10-2 damage: min 1, max 18 (avg 9.5)
5d4-2 damage: min 3, max 18 (avg 10.5)
4th range increment
2d10-3 damage: min 1, max 17 (avg 9)
5d4-3 damage: min 2, max 17 (avg 9.5)
5th range increment
2d10-4 damage: min 1, max 16 (avg 8.5)
5d4-4 damage: min 1, max 16 (avg 8.5)[/spoiler]
Not sure if you had done anything about short-barreled, but if we go with the 5d4 damage a sawed-off could lose one damage die per range increment (with a 30 ft. increment). Dangerous close up, but at 120+ feet, I think 1d4 damage sounds about right.